This invention relates to a thermosensitive coloring composition capable of reversibly assuming a color development state and a decolorization state depending upon the thermal hysteresis thereof and to a reversible thermosensitive recording medium using the above-mentioned coloring composition.
There is a known thermosensitive recording medium utilizing the coloring reaction between an electron donating compound (hereinafter referred to as a coloring agent) and an electron accepting compound (hereinafter referred to as a developer). This kind of thermosensitive recording medium is widely used in a variety of applications such as recorders and printers for an electronic computer, a scientific measuring instrument, a facsimile apparatus, a word processor, an automatic ticket vending apparatus and a CRT medical measuring instrument. The known thermosensitive recording media currently actually used are of an irreversible type in which the color development and decolorization cannot be repeated.
There are a number of proposals for a reversible thermosensitive recording medium utilizing a combination of a coloring agent with a developer and capable of reversibly assuming a color development state and a decolorization state. For example, JP-A-(Japanese Laid-Open Patent Application No.) 60-193691 proposes the use of a developer composed of a blend of gallic acid with fluoroglucinol and JP-A-60-237684 proposes the use of phenolphthalein or thymolphthalein as a developer. JP-A-62-138556, 62-138568 and 62-140881 suggest a reversible thermosensitive recording layer containing a homogeneous mixture of a coloring agent, a developer and a carboxylic acid ester. JP-A-63-173684 proposes the use of an ascorbic acid derivative as a developer, while JP-A-2-188293 and 188294 disclose the use of a higher fatty amine salt of gallic acid or bis(hydroxyphenyl)acetic acid as a developer. The known reversible thermosensitive recording media are, however, unsatisfactory for use in practice. In particular, with the known recording media, it is impossible to obtain both excellent color development state and excellent decolorization state or to obtain a constant image density even when the same recording condition is used.
JP-A-5-124360 and JP-A-6-210954 disclose a thermosensitive coloring composition containing a leuco compound serving as a coloring agent and an organic phosphoric acid compound, a phenolic compound or a carboxylic compound with a long chain aliphatic group as a developer, and a reversible thermosensitive recording medium utilizing the above coloring composition. The coloring composition can assume the color development state when heated at a first temperature and the developed color can be retained when rapidly cooled to room temperature. Further, the developed image can be erased when heated at a second temperature which is lower than the first temperature and the decolorization state is retained when cooled to room temperature. This thermosensitive recording medium gives a satisfactory image density. However, from the standpoint of practical use, there is a problem that the recording medium fails to give both high discolorization speed and good storage stability.